Because of the rapidly rising cost of natural gas, in recent years much effort has been directed to improving the efficiency of gas furnaces for residential and other consumers. Most modern gas furnaces use electronic ignition rather than a pilot light to avoid the inevitable waste of gas which a pilot light involves. Also, very high efficiency gas furnaces have been introduced; some of these furnaces use a pulse system in which pulses of a gas and air mixture are ignited within a combustion chamber, while others rely upon relatively complicated heat exchangers to extract the maximum amount of heat from the combustion products produced by burning natural gas. The latest furnaces are considerably more efficient than the older, pilot ignition gas furnaces; a typical pilot ignition gas furnace might have a steady state efficiency of about 75% and a seasonal efficiency of about 65%, while replacing the pilot light with either direct spark ignition of intermittent pilot ignition increases the seasonal efficiency to about 70%. Certain of the high-efficiency gas furnaces previously mentioned have seasonal efficiencies of about 92-95% under specialized conditions.
Although much attention has thus been directed to improving the efficiency of gas furnaces, many residential gas consumers have both a gas furnace for space heating and a gas water heater, and relatively little research appears to have been performed to improve the efficiency of gas water heaters. The design of residential gas water heaters has changed relatively little over the last few years. Most residential gas water heaters comprise a cylindrical water tank provided with an insulating jacket and a gas burner which impinges upon the base of the water tank; to allow escape of combustion products produced by the burning gas, a vertical conduit carrying the exhaust products extends vertically upwardly along the axis of the cylindrical tank, this vertical conduit serves to effect additional heat exchange between the combustion products and the water in the tank. The steady state and seasonal efficiencies of such gas water heaters are only about 70% and 55%, respectively, considerably lower than those of the high-efficiency types of gas furnaces previously described. Thus, the overall efficiency of gas consumption by most consumers is markedly diminished by the relative inefficiency of conventional gas water heaters. In fact, when appropriate weighting is given to the relative amounts of gas used by a heat efficient furnace and a conventional water heater in a typical household, the combined seasonal efficiency of gas usage is only about 55%. There is thus a need to improve the efficiency of gas water heaters in order to increase the overall efficiency of gas use.
The use of separate furnace and water heating units has other disadvantages in addition to the relatively low overall efficiency of gas use. The separate furnace and water heater require a relatively large amount of space and also require two separate pilot lights or other ignition systems, and separate gas lines, thus increasing installation costs.
In order to overcome the aforementioned disadvantages of separate gas furnaces and gas water heating units, it is desirable to provide a single unit which functions both as a gas furnace for space heating and as a gas water heater. This invention provides an apparatus which can function both as a gas furnace and as a gas water heater and also provides a method for burning a combustible gas which enables such a combined gas furnace and water heater to achieve higher efficiency. Finally, this invention provides a heat exchanger effecting heat exchange between a hot gas and a liquid which can be used in the aforementioned combined gas furnace and water heater.